1. Field of the Invention
The present invention relates to a dip-plating apparatus for steel strips or sheets, such as a Zn hot-dip plating of steel sheets, capable of remarkably improving the appearance of the plated surfaces.
2. Description of the Related Arts
In recent years, dip-plated steel strips or sheets are finding increasing use, both in fields and quantity, as is the case of application of alloyed Zn hot-dip plated steel sheets to outer panels of automobiles, as well as application of Al dip-plated steel sheets to various automotive parts. Consequently, requirements for higher quality of dip-plated steel strips or sheets are becoming more severe.
One of the critical requisites for the product quality is that the dip-plated surfaces of steel strips or sheets have an attractive appearance. In case of a zinc hot-dip plating, minute defects such as pimples are often observed in the plated surfaces of the plated steel sheets. Such defects undesirably impair the appearance of the plated sheets and, when the sheets are worked by, for example, a press subsequently to the plating, these defects are amplified to seriously degrade the appearance of the product after the work.
These defects are attributable to trapping of foreign matters in the plating layer, e.g., trapping of zinc oxides, Fe-Zn alloys and Fe-Al alloys contained in or floating on the surface of the plating bath in case of Zn hot-dip plating. These matters are generally referred to as "dross" hereinafter.
Hitherto, various methods and apparatus have been proposed for the purpose of eliminating defects due to trapping of dross. For instance, Japanese Unexamined Patent Publication No. 57-203764 discloses an art in which generation of oxide-type dross is suppressed by a seal box which surrounds the portion of a steel strip rising from the plating bath so as to control the oxygen concentration in the region around the rising portion of the steel strip.
On the other hand, a method has been proposed in, for example, Japanese Unexamined Patent Publication No. 62-202070, in which floating dross is removed by filtering or floatation.
The method proposed in Japanese Unexamined Patent Publication No. 57-203764, however, cannot produce any appreciable effect in removing influences produced by dross floating on the bath such as Fe-Al and Fe-Zn alloys, although it effectively suppresses generation of oxide-type dross.
The apparatus shown in Japanese Unexamined Patent Publication No. 62-202070 often suffers from clogging of filters, with the result that the operation becomes unstable. In addition, this art cannot produce any effect against floating dross.